The present invention relates to an optical fiber amplifier for an optical transmission system and an optical transmission system using an optical fiber amplifier. More particularly, the invention relates to an optical fiber amplifier for wavelength multiplexing and a wavelength multiplexing optical transmission system.
By the appearance of an optical fiber amplifier, a light signal having a weak light intensity can be amplified to light having a high output power with little noise. As a result, the optical transfer distance can be made much longer.
Further, since the optical fiber amplifier has a wide gain wavelength range from 1530 nm to 1565 nm, a wavelength multiplexing optical transmission in which a plurality of signal lasers within the amplification wavelength range are simultaneously amplified and transferred can be realized. For example, according to xe2x80x9c2.6 Terabit/s WDM Transmission Experiment using Optical Duobinary Codingxe2x80x9d (22nd European Conference on Optical Communicationxe2x80x94ECOC ""96 Postdated Line Paper Th. 3.1), it is realized that lasers of 132 wavelengths from 1529 nm to 1564 nm at the modulation rate of 20 Gb/s per wavelength are simultaneously transmitted over 120 km. In the announcement, an optical fiber amplifier for transmission compensates an optical loss occurring in a wave multiplexing part when the wavelength multiplexing is performed and has a function of increasing the output in order to make the transfer distance longer. An optical fiber amplifier on the transmission side obtains a light output of 21 dBm when the light of 132 wavelengths are simultaneously outputted.
In the wavelength multiplexing transmission, it is necessary to set an optical output of each signal wavelength between a lower limit optical output for keeping a signal-to-noise ratio at a necessary level and an upper limit optical output which does not cause a waveform distortion by a non-linear effect in the transmission line. On the other hand, in the optical fiber amplifier, a gain usually has wavelength dependency (gain deviation) and an output range between wavelengths is accumulated every relay and amplification. Since a signal error occurs when the range exceeds a permissible width of the optical output, it is necessary to suppress the gain deviation between wavelengths by the optical fiber amplifier.
As a method of controlling the optical output of the optical fiber amplifier at the time of wavelength multiplexing, there is a method of adjusting an optical output by an optical attenuator on the output side by executing a gain flattening control so that the optical output of every wavelength becomes constant irrespective of the degree of multiplexing as disclosed in xe2x80x9cEr:Doped Fiber Amplifier for WDM transmission Using Fiber Gain Controlxe2x80x9d (Technical Report of IEICE, OCS94-66, p. 31). In order to satisfy a gain flat condition, however, when an optical input increases, a large optical output from an optical fiber for amplification is requested and a strong pump optical power is accordingly necessary. In order to set the optical output within a predetermined optical output range, the increased optical output is decreased by an optical attenuator to an optical output equivalent to that at the time of low input level, so that it is not efficient.
In xe2x80x9cConfiguration Design of Multi-wavelength Er-doped Fiber Amplifier for WDM transmission Systemxe2x80x9d (Technical Report of IEICE, OCS95-36, p. 21), another configuration of the wavelength multiplexing Er-doped fiber amplifier is shown. With the configuration of two-stage amplifier, gain flattening control is performed by an amplifier at the front stage to thereby keep the gain constant irrespective of an optical input. In the optical fiber amplifier, since the wavelength strongly depends on gain, by executing the gain flattening control, it can be controlled so that the gain dependency of the wavelength does not depend on the optical input. In a post-stage amplifier, an optical attenuator is arranged in an input part and it is controlled so that the value of an optical input to the amplifying part is constant. In this manner, while maintaining the whole light gain to be constant, it is controlled so that the optical output is constant. Further, by designing so that gain tilt at the front and rear stages is cancelled, the whole gain flatness can be obtained without using an optical filter. The optical output is set so that the total optical output of all of the wavelengths is constant by decreasing an output of 7 dBm of each channel at the time of four-channel multiplexing to 1 dBm at the time of 16-channel multiplexing.
The optical fiber amplifier is used not only for amplifying a light output in the event of wavelength multiplexing but also for compensating a loss in a functional optical component. As the distance of the optical fiber transmission line having wavelength dispersion is increased, a dispersion value becomes larger. In order to eliminate an influence caused by the dispersion, it is necessary to compensate the wavelength dispersion. In xe2x80x9cDispersion-Compensator-Incorporated Er-Doped Fiber Amplifierxe2x80x9d (Optical Amplifiers and Their Applications 994 Technical Digest Series Vol. 14, p. 130), it is described that a wavelength dispersion compensator is incorporated as an optical function component in the center of an amplifier to compensate dispersion. When an optical component in which a loss occurs is arranged in the central part of an optical fiber amplifier which is divided into two parts, while keeping low-noise performance of the optical amplifier, a loss of the optical function component is seemingly reduced and a pumping efficiency can be increased.
In Japanese Patent Application Laid-Open No. 7-281219, an optical amplifier in which a variable optical attenuator is inserted into the front stage and an output distortion of an optical fiber amplifier is reduced is described. Further, in U.S. Pat. Nos. 5,500,756 and 5,555,477, a supervisory optical control is described.
When light of a number of wavelengths enters an optical fiber amplifier and an output level is controlled so as to be constant, an optical gain expressed by a ratio of an optical output and an optical input changes when the optical input changes. Since the wavelength dependency of gain of the optical fiber amplifier changes when the gain changes, the wavelength dependency of gain changes with the change in the optical input. There is no problem if interval lengths of a transmission line as intervals of providing optical fiber amplifiers are the same and a fixed optical input is supplied to each of the optical fiber amplifiers. In reality, however, there are various interval lengths and various optical input levels. Consequently, an optical fiber amplifier in which the wavelength dependency of gain does not change even if an optical input level changes is necessary. For example, in a reception optical fiber amplifier in which the maximum gain is 30 dB and the optical output is 0 dBm, the dynamic range has to correspond to a range from xe2x88x9230 dBm to xe2x88x929 dBm. In a relay optical fiber amplifier, the gain of about 40 dB is necessary. In a silica erbium-doped optical fiber amplifier, a gain deviation between 1535 nm and 1542 nm is every large and there is a change corresponding to the gain difference of about 6 dB between the minimum and maximum optical inputs.
A problem of the wavelength dependency will now be described with reference to FIG. 1. FIG. 1 shows a wavelength dependency of an optical output of an EDFA (erbium-doped fiber amplifier) when a total output level is controlled to be constant. A, B, and C in the diagram are arranged in accordance with the order from lower optical inputs. As the optical input increases, the optical output is markedly reduced around 1530 nm and is increased around 1560 nm. Even if an optical filter for correction is inserted so as to flatten the wavelength dependency of the optical output with a specific optical input, since the wavelength dependency characteristic of the gain changes when the optical input changes, the wavelength dependency of the optical output appears. When filters are connected at multiple stages, the deviation is accumulated by the number of stages. In a long distance transmission of 600 km or longer, four to seven relay optical fiber amplifiers are necessary and it is requested to suppress the output wavelength deviation per optical fiber amplifier to 1 dB or smaller.
According to the present invention, as a method of solving the problem, a variable optical attenuator is attached to an input terminal of an optical fiber amplifier in order to maintain input power level to an amplifying optical fiber to be constant. In case of inserting a variable attenuator on the input side of the optical fiber amplifier and controlling so as to maintain input power level to an EDF (erbium-doped optical fiber) to be constant, if a feedback control is executed too quickly, signal outputs of 0 and 1 in a low frequency region (about 10 kHz) of a digital transmission signal are averaged, so that a frequency characteristic of the transmission signal deteriorates. The optical fiber amplifier is a reproducer having a 1R (Reshape) function and should not limit a signal band. Consequently, it is necessary to control so as not to limit a band on a low frequency side of an amplifying optical fiber.
Further, in case of inserting the variable optical attenuator to the input side of the optical fiber amplifier and controlling to maintain the input level to the EDF to be constant, a monitor is disposed after the variable optical attenuator. Since the input to the EDF is controlled to be a constant value by the variable attenuator in the configuration, the actual optical input level to the optical fiber amplifier cannot be directly monitored. It is, however, necessary to monitor the input power level also in a state where the level of the input power is controlled by the variable optical attenuator.
It is desired to optionally change the use frequency in an operating state in the wavelength multiplexing optical transmission. It is necessary to always set the output level of the optical transmission system to be within a permissible range by an optical output per channel in the wavelength multiplexing optical transmission even if the number of transmission channels is changed.
In the foregoing related arts, optical output of each channel is controlled to be the same when all of the wavelengths are available in the wavelength multiplexing optical fiber amplifier. In an actual operation, however, all of the number of channels are not always available. There is also a case such that a small number of channels is used in the beginning and the number of channels is increased in accordance with necessity. It is desired to change a total optical output and to assure the minimum optical output when an optical input is changed.
Not only in the wavelength multiplexing system, in the optical fiber amplifier, an optical surge occurs on the optical output side when the level of the optical input suddenly increases in a short time of few xcexcs from a state where the optical input is low. It is a phenomenon peculiar to the optical fiber amplifier and is due to the property to keep the optical gain to be constant. An optical output having a high optical surge may destroy an expensive and important photosensing device. It is therefore necessary to suppress the optical surge. In case of executing the control to keep the optical output at a specific value, when the optical input is small, a potential gain of the optical fiber amplifier is increased. When a high optical input is suddenly received in such a state, an optical surge peculiar to the optical fiber amplifier occurs. In order to suppress the optical surge, it is necessary to suppress rise in the potential gain and to suppress optical pumping as a cause of the potential gain. As described in U.S. Pat. No. 5,355,250, there is conventionally a technique using a method of completely shutting down the optical pumping when an absolute value of the optical input decreases to a value equal to or lower than a specific optical input.
In the optical fiber amplifier operated in a wider optical input width, however, a specific optical input value at which it is discriminated to suppress the pumping is set to a low value. In this instance, when there is an optical input just higher than the specific optical input (that is, the lowest optical input within the operating range), a high pumping state is obtained. When the optical input is recovered to a high optical input in such a state, an optical output like an optical surge is generated. It is, therefore, desired to also suppress the optical output like the optical surge.
When an optical component is inserted into the optical fiber amplifier comprising optical amplifiers at a plurality of stages, there is another factor causing the optical surge. When an optical component is inserted in the optical fiber amplifier, there is a risk that the optical surge occurs when the optical component is again inserted from a state where the component is once detached. For example, a case of pumping optical fibers for amplification at the front and rear stages by one pumping light source in the foregoing xe2x80x9cDispersion-Compensator-Incorporated Er-Doped Fiber Amplifierxe2x80x9d will be described. Even if an optical input to the optical amplifier is a predetermined value, an optically disconnected state is obtained when the optical component inserted in the intermediate part of the amplifier is detached. Since no optical input is supplied to the rear amplifying optical fiber, the pumping is increased. When the optical component is again inserted in such a state, the signal light amplified by the front amplifier enters the highly pumped rear amplifying optical fiber, so that an optical surge occurs on the optical output side. It is necessary to suppress the optical surge at the time of re-insertion of the optical component into an intermediate part of the amplifier.
When the structure in which the optical function component can be attached and detached to/from the center of the optical fiber amplifier is used, it cannot be discriminated whether the reduction in the optical output of the rear amplifying optical fiber is caused by a failure or detachment of the optical component. A method of discriminating whether the optical output reduction is due to the detachment of the optical function component or a failure in the optical fiber amplifier is necessary.
Further, the construction of an optical system box for housing the optical system is necessary so as not to expose optical fibers and so as to eliminate a failure of an optical fiber disconnection due to bad handling. It is, however, necessary to assure a space for bend radius of the optical fiber necessary for reliability in the housing of the optical fiber, a vacant space occurs when a plurality of optical components are housed due to the necessity of the space for bending, and it causes increase in the size of the system. Consequently, a method of reducing the vacant space as much as possible is necessary to reduce the size of the optical fiber amplifier.
On the other hand, functional arrangement of pins of a semiconductor laser module is different according to manufacturers. It is necessary to properly use semiconductor laser pumping modules having various performances of various manufactures by a single circuit board in accordance with purpose and price from the view point of cost reduction. A pattern of circuit board corresponding to various pin arrangements is effective on suppressing of the quantity of stocks of various kinds of circuit board and elimination of waste in cost.
A cheap small optical fiber amplifying transmission system which always maintains necessary optical output level every wavelength at an optional degree of wavelength multiplexing and has no destructive element such as an optical surge is desired for an optical transmission system from the viewpoints of reliability, popularity, and transmission quality.
A method of inserting an optical attenuator into an optical input part of an optical fiber amplifier and inserting a fixed optical attenuator into an input part of an optical amplifier in accordance with an optical input level so as to set the optical input level to an amplifying optical fiber to be within a specific range is used.
In case of performing a long distance optical transmission, since a loss by an optical fiber transmission line increases, it is necessary to increase an optical output on the transmission side. In this instance, nonlinear self phase modulation or the like occurs in an optical transmission line and a waveform is changed. In designing of optical transmission, the light nonlinear phenomenon is considered. In order to improve wavelength flatness of the amplification factor of the optical fiber amplifier, it is necessary to regulate the optical input range. Consequently, when the optical transmission line is short, an optical attenuator has to be inserted into somewhere. According to the invention, the optical attenuator is inserted on the reception side, that is, near the input part of the optical amplifier. When the optical attenuator is inserted on the optical transmission side and the optical input level to the optical amplifier on the reception side is adjusted, the amount of nonlinear effect is changed and necessity to change the design numerical value every transmission distance is caused. However, by inserting the optical attenuator in front of the optical amplifier on the reception side in a manner similar to the invention, common nonlinear design is realized irrespective of the length of the transmission line and a common waveform characteristic can be used.
Further, as a method of improving the accuracy of the optical input range and eliminating the necessity to insert a fixed optical attenuator of different optical attenuation each time in accordance with a using state, a variable optical attenuator is inserted into an optical input part of the optical fiber amplifier to adjust the optical input level to the amplifying optical fiber to be constant. The attenuation of light is adjusted by monitoring the level of light just after attenuation and executing feedback control to the variable optical attenuator so that the monitored value becomes always constant.
The principle will be described. The wavelength dependency of gain of the optical fiber amplifier strongly depends on the gain of the optical fiber amplifier. This will be again described with reference to FIG. 1. FIG. 1 shows the wavelength dependency of gain when a wavelength multiplexed optical input is amplified in a lump and an optical output is controlled to be constant. Total optical inputs obtained by adding optical inputs of wavelengths are larger in accordance with the order of A, B, and C. The optical gain is expressed by the ratio of an optical output and an optical input. When the optical output is controlled to be constant, the gain becomes smaller by 10 dB each in accordance with the order of A, B, and C as the optical input becomes larger. When the optical input is small and the optical gain is large, the optical gain near 1530 nm rises remarkably high as compared with other wavelengths. When the optical input is high and the optical gain is small, the gain in a short wavelength zone is suppressed as C. The gain reduction becomes small on the long wavelength side.
In the optical transmission system, an optical output to the optical fiber amplifier is specified to a narrow width, for example, about xc2x11 dB from the viewpoint of use. Since an interval loss of an actual transmission line is not always constant, the optical input is changed by about 20 dB by the loss of set intervals. In the case where the wavelength range from 1530 nm to 1560 nm is used in the wavelength division multiplexing optical transmission and light in the wavelength range is uniformly amplified in a lump, if the optical input level is changed, the optical gain is changed, so that the wavelength dependency of the optical gain is changed.
According to the invention, by setting the range of the optical input to the amplifying optical fiber to be within a specific range, the wavelength dependency of the optical output can be suppressed to be relatively small. For example, when the optical input is higher than the predetermined range, by inserting a fixed optical attenuator, the optical input can be set to be within the desired optical input range.
By adjusting the attenuation so that the optical output from the optical attenuator is set to be constant in correspondence to the optical input of the optical fiber amplifier to make the optical input to the amplifying optical fiber constant, the accuracy can be further raised. Since the optical amplification amount in the amplifying optical fiber can be set to be constant, the wavelength dependency of gain does not change. In FIG. 1, the optical attenuator in the input part is adjusted always to be coincided with A having the largest gain (smallest optical input). With respect to the wavelength characteristic of gain, the gain is corrected by an optical filter or the like so that the wavelength dependency is flattened by coinciding the gain peak at 1535 nm and the gain near 1550 nm with the gain at 1540 nm. An optical fiber amplifier for wavelength multiplexing in which the wavelength dependency of gain is flat corresponding to a wide dynamic range of optical inputs can be constructed.
In the above method, by increasing the loss on the input side, the optical input to the amplifying optical fiber is adjusted. The optical input to the amplifying optical fiber is always set to be the minimum optical input. By the control, however, when the optical signal input is large, the noise characteristic deteriorates by the attenuation on the input side. A method of improving the drawback will be described. The dependency of the wavelength dependency of gain of an actual amplifying optical fiber on the optical input is not extremely large. The optical input serving as an adjustment reference is set to a point higher than the minimum optical input, for example, by 5 dB. By minimizing the attenuation of the optical attenuator in a range between the optical input higher than the minimum optical input by 5 dB and the minimum optical input, the drawback can be improved. The optical input to the amplifying optical fiber is changed within the range of 5 dB and a small wavelength dependency of about 1 dB is resulted.
A method of controlling the optical output in accordance with the number of channels at the time of wavelength multiplexing will be described. Channel number information on the transmission side is sent to the optical fiber amplifier over a supervisory signal. The optical fiber amplifier receives the channel number information, sets a signal voltage for controlling the optical input and output corresponding to the channel number information, and controls the optical input and optical output to/from the amplifying optical fiber.
In order to assure the minimum optical output of each channel, a total optical output is adjusted in accordance with the number of channels. The optical input level is changed by the transmission interval loss and the total optical input is also changed when a transmission output from a relay is changed in accordance with the number of channels. Since the wavelength dependency of gain depends on the gain of the amplifying optical fiber, it is also necessary to adjust the attenuation of the optical input in accordance with the channel number information so that the gain is set to be constant. For example, in case of 10 dBm (10 mW) at one wavelength, when information of two wavelengths is sent, it is increased by 3 dB (twice) to thereby set 13 dBm (20 mW). When the optical input per channel is xe2x88x9220 dBm (10 xcexcW), the optical input is increased by 3 dB (twice), thereby setting xe2x88x9217 dBm (20 xcexcW). In this manner, the optical output of each channel is set to be constant in spite of the change in the number of channels, change in gain of the amplifying optical fiber is eliminated, and the wavelength flatness is assured.
A method of preventing the optical surge will now be described. An optical signal fluctuation time of the system using the optical fiber amplifier is 1 m/sec or shorter (necessary band  greater than 1 kHz) and the fluctuation amount is 3 dB or less in the frequencies up to 1 MHz. When optical inputs are averaged in a time longer than time necessary for transmitting the optical signals by about two digits, in a stable state at the average optical input level in a system having therein the optical fiber amplifier, it can be regarded as constant. When the amount is reduced to a value (relative value) which is lower by, for example, 6 dB suddenly (yet,  greater than 1 xcexcs) within 1 m/sec as compared with the averaged optical input, it is regarded as an abnormal reduction in input. When the optical output is controlled to be constant by the optical fiber amplifier, at the time of reduction in optical input, the pumping is increased in order to maintain the optical output. When the optical input is suddenly recovered within 100 xcexcs in the highly pumped state, an optical surge occurs on the output side. In order to avoid the occurrence of the optical surge, at the time of the abnormal reduction of the optical input to the relative value, the pumping is suppressed to a level at which no optical surge occurs.
According to the invention, the optical input is averaged by monitoring light via a filter of a very low frequency in an analog circuit, storing a sample value of an ms order for about 1s in digital control, and performing the averaging process of the value. Data which is 1s ago is replaced by new data every sampling.
When an input is abnormally lower than the minimum optical input value of the system, the high pumping state is also caused. The high pumping state which cannot be considered as a normal use state has to be avoided. When the optical input gently decreases, the abnormal reduction in optical input by the relative value cannot be detected. Consequently, a loss of signal (LOS) is detected by the absolute value of the optical input and optical pumping is stopped. The LOS can be detected directly by an input monitor when there is no variable optical attenuator. When the variable optical attenuator is used, the LOS is detected by using both of a control signal of the variable optical attenuator and the optical monitor. By the pumping reduction control by the OR logic value between the relative LOS and the absolute LOS detection value, the occurrence of the optical surge is suppressed.
With respect to the optical input monitor used for discriminating the optical input, when the variable optical attenuator is inserted on the input side, the optical input cannot be monitored. As a counter measure, a method of directly monitoring the optical input by arranging an optical tap coupler for monitoring the optical input is installed before the variable attenuator can be considered. There is, however, a problem of deterioration in the optical signal due to insertion of the optical input monitor. As another method of monitoring the optical input while avoiding the drawback, there is a method of obtaining an actual optical input by adding the attenuation obtained from the control signal of the variable optical attenuator and an optical monitor value just after the variable optical attenuator.
A method of suppressing the occurrence of an optical surge caused by re-insertion of an optical function component when the optical function component is inserted into an intermediate part of the optical fiber amplifier comprising optical amplifiers of a plurality of stages. An optical monitor is inserted on the input side of the optical amplifier at the post stage. When the optical monitor detects a value lower than a predetermined value, it is discriminated that the optical component is not connected and the intensity of pumping is reduced. Consequently, the highly pumped abnormal state of the rear amplifier is suppressed and the occurrence of the optical surge at the time of connecting the optical component is suppressed. If the pumping state at the front stage and that at the post stage are independent, the optical output of the pumping light source at the post stage is reduced when the optical component is not connected.
As a configuration of the optical fiber amplifier having optical amplifiers at a plurality of stages, there is a one-pumping and two-stage amplification configuration in which residual pumping light of the front optical amplifier is used for the post optical amplifier. In this case, the optical output of the front optical amplifier is monitored and also the optical output of the rear optical amplifier is monitored. When there is an optical input to the post stage, the pumping is controlled so that the optical monitor at the post stage becomes constant. When there is no optical input to the post stage, the control mode is switched so that the optical output monitor at the front stage becomes constant by low pumping which does not cause an optical surge at the time of connection of the optical component. By using weak light amplified by the front optical amplifier when the optical component is connected, the input to the rear optical amplifier at the time of optical component connection is detected, and the optical monitor at the post stage automatically detects light and the optical output of the post stage is controlled to be constant.
Insertion of the optical component, detection of un-insertion, information transmission, and process inherent modulation band of the optical fiber amplifier, thereby enabling frequency band deterioration when the variable optical attenuator is used to be suppressed. The band of the optical fiber amplifier is usually few kHz or lower and is sub kHz when it is slow. Even if the magnetooptical variable optical attenuator is used, the band control by electric control is necessary. When the optical input is adjusted by the variable optical attenuator, the optical input level is controlled at a slow speed so as not to disturb the band of the amplifying optical fiber.
Mounting will be described. The optical fiber amplifier is constructed by a plurality of fiber type optical components such as amplifying optical fiber, combiner of signal light and pumping light, optical isolator, and optical monitor. For the optical fiber, a bend radius R greater than 30 mm is required in order to assure the reliability for bending. That is, in order to arrange the optical fiber, minimum 60 mm bend space is necessary. When an elongated optical component is arranged along the flow of the optical fiber, it is necessary to add the bend space of the optical fiber to the length of the optical component. For example, for an optical component having the length of 70 mm in will be described. In the monitoring system, when the optical output monitor at the front stage shows a predetermined value and the optical input monitor at the post stage does not show the optical input, it means that the optical component in the center is not inserted. In this case, information of un-insertion is transmitted. When this signal is sent, even if information of abnormal reduction in optical output of the optical amplifier is transmitted, it is not the reduction in optical output due to failure of the optical amplifier, output abnormality is masked.
Control frequency characteristics will be described. When the optical input is controlled, it is preferable to use a magnetooptical variable optical attenuator as described in xe2x80x9cpreliminary report C-128 for 1996 IEICE electronics society conferencexe2x80x9d, which has response speed of about 300 xcexcs. When the response speed is maximally used, a frequency range of about 3 kHz can be used. When an electric absorption type modulator is employed, the optical input can be controlled in a time of sub ns. When an acousto-optical modulator is used, it can be controlled in a time less than xcexcs. The low frequency side of the output constant control band of the variable optical attenuator is controlled to be slower then the the fiber coupling direction, bending of the fiber of R=30 mm is added to each of both sides. The length of minimum 130 mm is therefore necessary. When the optical component is obliquely arranged, it becomes shorter when seen from one side of a rectangle. For example, when the optical component is inclined by 30xc2x0, 70*cos(30xc2x0)=60.6 mm. The fiber bend space 60 mm is added to 60.6, so that the necessary length is 120.6 mm which is shorter by 9.4 mm. When the optical component is inclined by 45xc2x0, the necessary length is 109.5 mm which is shorter by 20.5 mm. Further, by mounting with the fiber bend space, high density mounting to a realistic space can be realized.
Wiring corresponding to pin arrangement of various pumping lasers will be described. First, wiring corresponding to the pin arrangement of an available semiconductor laser module for pumping is preset in a circuit board pattern. The wiring are connected to commonly connecting pins by pad holes. By connecting pins of different arrangements according to various pumping lasers via a 0 ohm chip resistor or a short-circuiting jumper to pads, the wiring is terminated by open pads so that pins can be selectively connected. Pads of corresponding pins are terminated by open pads at the positions corresponding to the open pads.
As mentioned above, all of possible wiring is arranged by open pads for pins having functions different depending on a pumping laser. In actual use, the open pads corresponding to the function of a pumping laser used are connected via 0 ohm chip resistors or jumper wiring. In stead of open pads, wiring corresponding to possible functions of various kinds is connected to hole pads, the length is adjusted including the bend of pins of a semiconductor laser module, and pins are connected to pads of an adapted pattern, thereby corresponding to different pin arrangements.
By making the circuit board pattern common so as to correspond to a pumping laser at the time of production, the laser module can be changed to a laser module which is easy to use at low cost.
By using the above-mentioned optical fiber amplifier for the wavelength division multiplexing optical transmission system, even if the number of channels is changed, the minimum optical output per channel is kept, an optical function component can be inserted into an intermediate part, an optical surge is suppressed, a small system can be constructed at low cost, transmission quality is maintained, and compensation function of wavelength dispersion and the reliability of the transmission system is enhanced.